Some race condition fixen.

cmd.go

@@ -119,7 +119,7 @@ func main() {
 		}
 
 		if count > 0 {
-			b = append([]byte(","), b...)
+			b = append([]byte(",\n"), b...)
 		}
 
 		_, err = w.Write(b)

crawler.go

@@ -21,7 +21,7 @@ type Crawler struct {
 	PeerAge        time.Duration
 	ConnectTimeout time.Duration
 	shutdown       chan struct{}
-	waitGroup      *sync.WaitGroup
+	waitGroup      sync.WaitGroup
 }
 
 type Result struct {
@@ -30,24 +30,29 @@ type Result struct {
 }
 
 func NewCrawler(client *Client, seeds []string) *Crawler {
-	var wg sync.WaitGroup
-
 	c := Crawler{
 		client:     client,
 		seenFilter: map[string]bool{},
 		shutdown:   make(chan struct{}, 1),
-		waitGroup:  &wg,
+		waitGroup:  sync.WaitGroup{},
 	}
 	filter := func(address string) *string {
 		return c.filter(address)
 	}
-	c.queue = queue.NewQueue(filter, &wg)
+
+	done := make(chan struct{})
+	c.queue = queue.NewQueue(filter, done)
 
 	// Prefill the queue
 	for _, address := range seeds {
-		c.queue.Add(address)
+		c.addSeed(address)
 	}
 
+	go func() {
+		c.waitGroup.Wait()
+		done <- struct{}{}
+	}()
+
 	return &c
 }
 
@@ -120,6 +125,13 @@ func (c *Crawler) handleAddress(address string) *Result {
 	}
 }
 
+func (c *Crawler) addSeed(address string) {
+	c.waitGroup.Add(1)
+	if c.queue.Add(address) == false {
+		c.waitGroup.Done()
+	}
+}
+
 func (c *Crawler) filter(address string) *string {
 	// Returns true if not seen before, otherwise false
 	c.numSeen++
@@ -142,10 +154,7 @@ func (c *Crawler) process(r *Result) *Result {
 			continue
 		}
 
-		c.waitGroup.Add(1)
-		if !c.queue.Add(NetAddressKey(addr)) {
-			c.waitGroup.Done()
-		}
+		c.addSeed(NetAddressKey(addr))
 	}
 
 	if len(r.Peers) > 0 {

queue/queue.go

@@ -9,14 +9,14 @@ type Queue struct {
 	filter  func(string) *string
 	count   int
 	cond    *sync.Cond
-	waitGroup *sync.WaitGroup
+	done    <-chan struct{}
 }
 
-func NewQueue(filter func(string) *string, waitGroup *sync.WaitGroup) *Queue {
+func NewQueue(filter func(string) *string, done <-chan struct{}) *Queue {
 	q := Queue{
 		storage: []string{},
 		filter:  filter,
-		waitGroup: waitGroup,
+		done:    done,
 	}
 	q.cond = sync.NewCond(&q)
 
@@ -24,12 +24,13 @@ func NewQueue(filter func(string) *string, waitGroup *sync.WaitGroup) *Queue {
 }
 
 func (q *Queue) Add(item string) bool {
+	q.Lock()
 	r := q.filter(item)
 	if r == nil {
+		q.Unlock()
 		return false
 	}
 
-	q.Lock()
 	q.storage = append(q.storage, *r)
 	q.count++
 	q.Unlock()
@@ -42,7 +43,7 @@ func (q *Queue) Iter() <-chan string {
 	ch := make(chan string)
 
 	go func() {
-		q.waitGroup.Wait()
+		<-q.done
 		q.cond.Signal() // Wake up to close the channel.
 	}()